The invention relates to a method for determination of functional residual capacity (FRC), by introduction of helium or another inert gas, using an oscillatory or other determination, wherein a gas mixture containing helium or another inert gas is delivered to a patient or subject in an open system via a respirator or another appliance supporting respiration, and the concentration and quantity of this gas mixture and thus the foreign gas or helium concentration is measured by way of a measurement apparatus connected to the tube or to the mask.
To be able to perform ventilation which does not impose a strain on the patient, it is necessary to know the patient's FRC. In this way the ventilation parameters can be adapted to the patient, the effect of which is that, during ventilation, the patient suffers less damage than when operating with too small or too great an FRC. The physiologically abnormal ventilation is thus mitigated, and the period required to wean the patient off the ventilator is substantially shortened.
It is already known to perform FRC determination by means of oscillatory density measurement, although in the previously known methods this can only be done in a closed system and, consequently, not during mechanical ventilation.
From DE 29 12 391 B2 it is known to carry out analysis of pulmonary function using an appliance with a respiratory bag, where the subject is connected up to the respiratory bag and, by breathing out and rebreathing a test gas mixture containing helium, the change in the density of the gas mixture in the respiratory bag is measured, and from this the FRC is calculated. This involves a closed system in which the subject breathes out into a closed bag and then breathes in again from this bag. For the determination of the FRC, the subject is connected up to the respiratory bag and, after a relatively short period of rebreathing, is again withdrawn from this system. FRC determination during mechanical ventilation is not possible with the known system. U.S. Pat. No. 4,221,224 describes how, in a method for determining the alveolar ventilation, the inspiratory part and the expiratory part of the system are separate from one another. Using the method disclosed, it is not the FRC which is determined, but gas values in the blood.